File upload to custom folder fails when using Themosis framework

[martinkrcho]

Issue Overview

File upload to custom folder in uploads folder is not allowed when using Themosis framework.

Steps to Reproduce (for bugs)

1. Create file upload field and set "upload_dir" parameter to upload files to uploads/acme-import
2. Try to upload a file
3. File upload fails

Expected Behavior

File upload should succeed.

Current Behavior

File upload fails.

Possible Solution

Add a filter to RWMB_File_Field::handle_upload_custom_dir to allow third parties to override the following check:

// Make sure upload dir is inside WordPress.
$upload_dir = wp_normalize_path( untrailingslashit( $field['upload_dir'] ) );
$root       = wp_normalize_path( untrailingslashit( ABSPATH ) );
if ( 0 !== strpos( $upload_dir, $root ) ) {
	return;
}

Default Themosis setup gives me something like this:
$root - /var/www/my-themosis-site/htdocs/cms
$upload_dir - /var/www/my-themosis-site/htdocs/content/uploads

I suggest to add the following filter to allow Themosis developers to implement exception for this.

// Make sure upload dir is inside WordPress.
$upload_dir = wp_normalize_path( untrailingslashit( $field['upload_dir'] ) );
$root       = wp_normalize_path( untrailingslashit( ABSPATH ) );
$inside_wp = (0 === strpos( $upload_dir, $root ));
if ( ! apply_filters( 'rwmb_file_inside_wp', $inside_wp, $upload_dir, $root) ) {
	return;
}